WASHINGTON - Scientists have developed a model to predict the movement of millions of people from one country to another. Experts say such projections are important because they affect social services - such as schools, jobs and health care - in countries that receive large numbers of immigrants.

Scientists have developed a model they say will dramatically improve the difficult task of tracking the migration of people across international borders. The United Nations has asked population researcher Joel Cohen and colleagues to create a formula to measure the ebb and flow of populations across borders, so countries can plan decades into the future.

Cohen offers this example of the challenge he and his colleagues faced. “The next time you are in a room with, let’s say, 50 people in it, try to count them. If there are people coming in and out of the room while you are doing it, it’s really hard to figure out at any given time how many people were in the room. The next time you are at a concert or a lecture just try it. And when you have a country of a couple hundred million [people] – 300 million [people] - like the U.S., it’s really hard,” he said.

While the United States has strict immigration policies, Cohen, a professor of population studies at Columbia University, says the country has no method of keeping track of people once they enter legally and of citizens who leave. Many other countries have no immigration or emigration data.

Cohen says current methods for determining global population migrations are flawed. He says that under some systems, the number people leaving a country can be greater than the population of the country they are leaving.

Other models use economic statistics, like new job creation and income levels. But Cohen says such information looks backward and does not project the influx of immigrants across borders decades into the future.

To get a more accurate picture of human migration, Cohen and his colleagues at The Rockefeller and Columbia Universities in New York and Imperial College in London analyzed more than 46,000 United Nations population reports of travel from 11 countries between 1960 and 2004.